A new approach to high resolution measurements of structure in superconducting tunneling currents

The design of electronic circuitry is described that has the capability of measuring independently the bias dependence of the first derivatives dI/dV (dynamic conductance) and dV/dI (dynamic resistance) and the second derivatives d2I/dV2 and d2V/dI2 of a tunneling current-voltage (I-V) characteristic over a wide range of junction impedances (less than 1 Ω to greater than 104 Ω). Connections to the sample are four terminal so that problems with lead resistance are eliminated. Resolution in the measurement of the first derivatives is better than one part in 104, and problems with long-term drift are obviated by the use of feedback. Modulation levels at the sample junction, both at constant current and constant voltage, are conveniently monitored with a calibrated lock-in amplifier and can be easily changed. The circuitry has been operated successfully with voltage modulation levels across the junction ranging from 1.0 μV to 2.5 mV rms. Data are presented on Al–I–Pb and Al–I–AuPb3 tunnel-junction samples to illustrate the capabilities of the system.